Several devices have been provided for measuring flow characteristics of a fluid. For instance, thermal apparatuses for measuring flow amounts have been designed. In these devices, a flow sensor generates heat, and the amount of heat radiated from the flow sensor to fluid is detected to thereby measure the flow amount. (See, e.g., U.S. Pat. No. 6,973,823 (claiming priority to Japanese Patent Publication No. 2004-53600), U.S. Pat. No. 6,938,473 (claiming priority to Japanese Patent Publication No. 2003-214915), U.S. Pat. No. 5,485,746 (claiming priority to Japanese Patent 3240782), U.S. Pat. No. 4,709,581, and U.S. Pat. Nos. 5,571,964 and 5,581,026 (claiming priority to Japanese Patent Publication No. Hei 5(1993)-164585). However, these and other related prior art devices have certain disadvantages.
For instance, in the device of U.S. Pat. No. 6,973,823, the axis of the measurement passage is curved. As such, the flow of the fluid may become uneven, making measurement inaccurate.
The apparatus described in U.S. Pat. No. 6,938,473 includes an obstruction member formed in a columnar shape, and air within a narrow area of the pipe flows into a sensor channel. As such, the flow velocity within the sensor channel is less likely to correlate with the flow velocity in the main portion of the intake port. For this reason, the flow measurements may be inaccurate.
Furthermore, the apparatus described in U.S. Pat. No. 5,485,746 has an inlet portion open toward the upstream side. As such, dust may flow in and stick to the flow sensor, and this makes measurement results unstable.
Moreover, the air flow meter illustrated in FIG. 26 of U.S. Pat. No. 4,709,581 includes a deflector that is smaller than the inlet opening. This allows dust to flow into a bypass channel, and this makes measurement results unstable.
Also, the devices described in U.S. Pat. No. 5,571,964 and U.S. Pat. No. 5,581,026 each include an inlet portion open toward the upstream side. As such, dust may flow in and measurement results may be unreliable.
In addition, flow sensing devices with thermal sensors (e.g., hot wire type mass air flow sensors) can be inaccurate. For instance, these devices often include one or more support members to which the thermal sensor is coupled. However, heat may be lost to the support member instead of the fluid. This may be especially true when the flow velocity of the fluid is relatively low (e.g., when an engine is idling). As such, the ratio of heat radiated to the fluid is reduced, and the ratio of heat loss is relatively increased. This significantly degrades the detection accuracy.
The devices in U.S. Pat. Nos., 5,485,746, 4,709,581, 5,571,964 and 5,581,026 include a central member (i.e., a first channel portion) and a main passage. The central member is enlarged to obstruct size of the main passage. The flow velocity of fluid flowing through the main passage is thereby increased. Also, the outlet portion of a bypass passage (i.e., a first channel) is obstructed by a downstream member or the downstream portion of the central member. Thus, the flow velocity of fluid flowing through the bypass passage is increased by the negative pressure produced by fluid flowing through the main passage. When the flow velocity is increased, the ratio of heat escaping to air is enhanced; therefore, the detection accuracy can be enhanced. However, the main passages respectively form part of an intake port. Thus, to obstruct the main passage, the central member is enlarged so that the outer wall surface of the central member is adjacent the inner wall surface of the intake port. That is, to enhance the detection accuracy of these devices, the central members are enlarged.